How do you know what to sub for yarn gauge?

[cinnybuns]

I'm currently looking at a pattern using caron yarn from joann's which comes out to 4"=18 stitches. What type of yarn can I use to substitue, and is there a website where I can refer back again?

[suzeeq]

Try yarndex.com. You can put in the gauge and it will list yarns with the same sts/inch. Most yarn in that gauge will be worsted weight.